First, neither tile nor grout is totally waterproof, so whatever you use in a shower must not be damaged by moisture. Typically, people would use cement board on the walls with a vapor barrier behind it since none of those components are waterproof, BUT, none of them are damaged by being wet. The vapor barrier prevents any moisture trapped in those materials from being in contact with the wood studs and structure. The same thing is true on the floor of your shower...there's a waterproof layer preventing water from getting to it.

So, it's not a long stretch to realize that if you do put drywall in a shower, if must be kept from ever getting wet or you'll have problems. This is exactly what the Kerdi membrane does...it is a very effective waterproofing layer. Just like the liner in your shower keeps the wooden subfloor from rotting out or ever getting wet, the membrane does it for not only the walls but the floor. So, if you built it properly - the drywall stays dry, the floor stays dry, and everything works out well.

As with anything, if you make a mistake or breech the waterproof layer, you'll have problems. But, after installation and a flood test, the entire surface is covered with tile, so that layer remains well protected, and is easily able to do its job.

Some related concepts: why can you use wood underneath your roofing shingles? Because when properly installed, the roof doesn't leak. Why can you use drywall on the inside walls next to your windows? Because a properly installed window doesn't leak. Sure, you can break the window or have wind blow a shingle off, but that's hardly likely to happen inside of a properly built shower covered in tile!

Schluter has ben using Kerdi over drywall for decades, and since the certification doesn't last forever, has been getting it retested to reverify it still works to stay in compliance bi-annually. The ICC (International Code Council) test report ESR-2467 covers the certification for both Ditra and Kerdi and is only valid if you follow the manufacturer's installation instructions. While other methods might work over drywall, to my knowledge, none have the history or the certification, or test data to show that they work properly in that environment. So, do not take Schluter's experience with Kerdi make you think that another system is certified, tested, and warranted in other than their tested, approved substrates.

FWIW, that report is valid over the following surfaces in a shower (from section 2.0):
"Kerdi is intended for use on floors over concrete, mortar and tile backerboard substrates. Kerdi is also used as shower lining, as required in IRC Section P2709.2 and IPC Section 417.5.2, over concrete, mortar, tile backerboard, expanded polystyrene foam board, prefabricated polystyrene shower trays, curb and ramp, gypsum board and masonry substrates."

So, you can choose what board you want in your shower when using Kerdi. Schluter's recommendation is drywall since it is available in larger sheets so you have fewer seams, accepts thinset well, and allows excess moisture from the thinset to be purged while maintaining a suitable environment for the thinset to cure and attain its best strength.

Kerdi will work equally well with any of the approved substrates - choose the one you want, install it per the manufacturer's instructions and you'll have a long-lasting, problem-free shower. Don't let any nay-sayers force you into any other extra steps by making you feel guilty...IF you do it right, it will not leak. IF it does, fix it before you put up your tile. Based on the certification, doing it otherwise could void the warranty and the certification of the Building Codes.

First, neither tile nor grout is totally waterproof, so whatever you use in a shower must not be damaged by moisture. Typically, people would use cement board on the walls with a vapor barrier behind it since none of those components are waterproof, BUT, none of them are damaged by being wet. The vapor barrier prevents any moisture trapped in those materials from being in contact with the wood studs and structure. The same thing is true on the floor of your shower...there's a waterproof layer preventing water from getting to it.

So, it's not a long stretch to realize that if you do put drywall in a shower, if must be kept from ever getting wet or you'll have problems. This is exactly what the Kerdi membrane does...it is a very effective waterproofing layer. Just like the liner in your shower keeps the wooden subfloor from rotting out or ever getting wet, the membrane does it for not only the walls but the floor. So, if you built it properly - the drywall stays dry, the floor stays dry, and everything works out well.

As with anything, if you make a mistake or breech the waterproof layer, you'll have problems. But, after installation and a flood test, the entire surface is covered with tile, so that layer remains well protected, and is easily able to do its job.

Some related concepts: why can you use wood underneath your roofing shingles? Because when properly installed, the roof doesn't leak. Why can you use drywall on the inside walls next to your windows? Because a properly installed window doesn't leak. Sure, you can break the window or have wind blow a shingle off, but that's hardly likely to happen inside of a properly built shower covered in tile!

Schluter has ben using Kerdi over drywall for decades, and since the certification doesn't last forever, has been getting it retested to reverify it still works to stay in compliance bi-annually. The ICC (International Code Council) test report ESR-2467 covers the certification for both Ditra and Kerdi and is only valid if you follow the manufacturer's installation instructions. While other methods might work over drywall, to my knowledge, none have the history or the certification, or test data to show that they work properly in that environment. So, do not take Schluter's experience with Kerdi make you think that another system is certified, tested, and warranted in other than their tested, approved substrates.

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If we have to look at an engineering POV , using the drywall in showers or wet areas is a no-no . The codes are a minimum -- repeat a minimum -- of building procedures to respect . Kinda strange to see you coming so strong with it as a recommendation .

Drywall compared with plywood is by far an inferior product . And talking about roof leaks -- most of the showers are build under roofs -- , if the tiled ceiling gets wet , the drywall will collapse . If the walls are getting wet ...

Quote :
" Moisture Movement (Wicking) within Gypsum Wallboard
Paper # 580
Dale J Greenwell and Marc Y Menetrez
US Environmental Protection Agency, 109 T.W. Alexander Drive, Research Triangle Park, NC
27711
ABSTRACT
Gypsum wallboard with repeated or prolonged exposure to water or excess moisture can lose
its structural integrity and provide a growth medium for biological contaminants. Poorly sealed
buildings, leaking or failed plumbing systems, or improperly constructed HVAC systems can all
contribute to water and moisture problems. Gypsum wallboard readily absorbs moisture through
direct contact with standing water and differences in water vapor pressure. Regular gypsum
wallboard sample sections were hung vertically and exposed to a continuous source of standing
water 1.3 cm in depth. Both water absorption (measured in percent moisture content) and
vertical movement (wicking height) were monitored within the wallboard over several days. The
moisture content measurements revealed a water movement pattern similar to paper
chromatography. A leading edge of relative high percent moisture content (~ 20%MC) moved
upward along the width of the wallboard. The immersed portion of the wallboard maintained a
moisture content level ≥ 20%. Between the leading edge and immersed portion of the wallboard
was an area of moderate moisture content (~ 12%MC). Water wicked to a height of 15 cm
within the first three hours of testing after which the rate continued asymptotically appearing to
reach a maximum height by day 16.
IMPLICATIONS
Most wallboard remediation techniques involve visual inspection and moisture content
measurements to determine the extent of the water damage and the presence of or potential for
mold growth. Remediation not performed within a reasonable time following discovery, may
result in ineffective remediation as moisture may continue to move beyond the original detected
region. Because of the wide spread used of gypsum wallboard in commercial and residential
construction, understanding the moisture movement (wicking) of water in gypsum wallboard
products is essential. ''

End quote .

If you want to rely only on the membrane -- Kerdi or similar -- for waterproofing and not consider the safety of the assembly , I think you are doing a good job .

No argument that when drywall gets wet, it loses integrity. But, it is no more likely to get wet in a properly built Kerdi shower than it is on the ceiling under a solid roof or next to a window or a door. The point being - Kerdi is proven waterproof when installed properly. Therefore, any tested and approved material used underneath it can produce a reliable, long-lasting shower. Use any one of the approved materials and it still works fine. When installed properly, the only way drywall (gypsum board) will get wet is from some other way, and that's not a problem with the shower build.

Once the shower is flood tested, then covered with tile, it is going to take some other fault in the house for it to cause a problem in the shower. I don't have any issue about which substrate you end up deciding to use, I have an objection to the years of test data that says it works, passes building codes, and is warranted by the manufacturer that has a vested interest in it working being touted as wrong.

Just in case the roof leaked, do you put CBU on the ceiling? What about next to a window or a door? No, you accept that whoever put the roof up or the door or window in did it properly, and if you have a fault, you fix it.

If you're really paranoid, you could make your house entirely out of cement, stone, and tile...then, if it got wet because of some misstep, you'd just mop up.

I guess you are working only on the membrane level and the rest is not important . Drywall , like other products, are only accepted by Schluter for a shower installation . None of the drywall mfgs recommends the use of drywall in wet areas.

I guess long lasting is less important than your coating of inferior products. Reminds me of the arguments that drywall contribute to increase the structural #s when calculate them . Probably they will also accept an inferior product saying that if everything else is done right , the walls will not buckle. Such a mediocrecy .

What happens to the wooden structure of the room when a shower pan leaks? The subfloor rots, the floor plate rots, the studs and maybe the joists rot.

If we trust Kerdi to protect the floor from getting wet, why can't you trust it to protect the walls?

Millions of square feet of tile has been successfully installed on drywall, so the bond to that substrate is a known thing.

If you believe Kerdi is waterproof, tests have shown it to be, why would you believe it can work as a pan liner and not also protect the walls it is installed on?

Agreed, drywall in a conventional shower is stupid. A Kerdi shower is no more a conventional shower than a salt crystal is a diamond.

If you don't trust yourself to install Kerdi properly, like apparently John Whipple, you use all of the extras you can think of to patch it and provide secondary layers of protection. Most people believe their roof will keep the house dry, the windows will keep out the rain, as will the doors when properly installed. People don't give a second thought to the use of drywall underneath the roof - look at the millions of square feet of cathedral ceilings, or the millions of homes with drywall next to the windows and doors.

It is not such a great leap to believe that neither the walls nor the floor of a properly built Kerdi shower will ever get wet. If it did fail, regardless of the materials, it won't make a difference what's there...you'd have to tear it out and repair it. So, build it right the first time...it works.

Explain to me why if Kerdi works as a pan liner, and one does not consider the subfloor under a shower a wet area, why is drywall behind a waterproof membrane that has been tested and approved for use in a shower, that drywall is in a wet area?

The whole point of this is that when installed properly, NOTHING on the other side of Kerdi is in a wet area.

If you do not believe Kerdi is waterproof, why would you ever use it as a pan liner?

I know it's wrong and the only reference you can hang on to is that Schluter says it's OK.

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Do you not put any credibility to the ICC? If when they tested if it failed, would they risk their standing and reputation? What about the hundreds of thousands of Kerdi showers that have been built that don't leak? ANY shower can fail if there are workmanship problems, and if it does, it really doesn't matter what's behind the waterproofing...it has to come out to be fixed.

As an engineer (I don't think that matters), and as a pratical person, I can't for the life of me understand why someone would usee drywall in a shower! If you had just a teeny tiny leak - the drywall is going to go to shit. Cement board could tolerate it, and maybe with no ill effects.

What do you save by using drywall? $50? Why risk it? What's the point? Because it can be done?

Notice that the original title says why you CAN use it in a shower...it doesn't say you HAVE to...

But, why? For the same reasons I do not worry about drywall on my ceiling, or next to my doors or windows. A leak there will have the same effect-tear out and fix. But, I've lived in this place for nearly 30-years, and I've never had to replace any drywall because of water leaks through the roof or from around windows or doors. Drywall is readily available in bigger sheets, cuts easily without carcinogenic dust so it has fewer joints that have to be supported with blocking, unlike the typical 3x5' sheets of cbu (yes, you can special order bigger sheets, but then try to get them to the job site unbroken and then you still have to lift them!). Over those 30-years, I've not had a water supply pipe fail where it leaked and caused damage. Why would you expect it to do it in your shower? There are seals made for protecting any penetrations through the Kerdi membrane for around things like the shower arm, and valves. Or, KerdiFix works, too. There is no reason why the drywall behind the shower wall is any more likely to get wet than in the rest of the house IF you build it properly.

Ain't gonna get in a pissin match with you fellas, but drywall and other gypsum boards, as a backer, have been used successfully in showers since it came out. See: mud shower. Mud showers with asphalt impregnated building paper, or plastic over the drywall work just fine and have since before I was born, over 50 years ago.

Schluter has a proprietary system that has been tested to meet or exceed industry standards. Plain fact is, their system works. You wanna not trust the manufacturer or industry who approves their products for their intended uses, when there are successful installs over drywall functioning just fine for many years out there, that's certainly your prerogative.

Schluter has a proprietary system that has been tested to meet or exceed industry standards. Plain fact is, their system works. You wanna not trust the manufacturer or industry who approves their products for their intended uses, when there are successful installs over drywall functioning just fine for many years out there, that's certainly your prerogative.

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IMO sites like this are frequented by DIY'ers who don't do this every day. It's my opinion that Kerdi will be more challenging for a person doing it for the first time, and maybe the only time.

IMO sites like this are frequented by DIY'ers who don't do this every day. It's my opinion that Kerdi will be more challenging for a person doing it for the first time, and maybe the only time.

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More challenging in what way? A conventional shower's liner requires some very detailed steps to ensure the liner is waterproof where you MUST cut it to fit over the curb, then seal corners on it, and maybe seam it if the sheet isn't big enough. The biggest worry about water leaks are at the pan and over the curb - gravity just dictates that the water is going to go there. Flood test it before you cover it with tile, just like a Kerdi build...fix it if it leaks. Especially on a wall, after the thinset cures, water does not wick very far into a seam, and if you build it correctly, you have at LEAST 2" of overlap. Prior to full cure, yes, it can weep in further, but even after a day, if you did it right, the pan does not leak. If you haphazardly put holes in your curb, you're an idiot, and anything that happens as a result, regardless of whether it is Kerdi on it or a conventional liner, you're going to have problems.

As to a surface, paint on waterproofing...I'll take a manufactured waterproof sheet any day over a waterproofing material I have to paint on and worry if it is without pinholes or is the proper thickness over the entire area to do its job properly.

My only thought would be this. If I build two showers for someone, both on the second story of a two story house, a master and a mail bath. Lets say that I used kerdi over drywall in one, and kerdi over CMU in the other. A year after I finish, I get a call: "Jim, we have spots on the ceiling below both showers you built". Is my response: "Oh, we will have to gut & re-do the one I drywalled, but the other will be fine because we used CMU".

I'm trying to figure out your logic that a water resistant material behind a waterproof membrane is better, because in a surface applied waterproofing system, if water gets past it...it's a total failure no matter what's behind it.